Objective: We aimed, in this study, to compare dispersion of the p wave in patients with type I diabetes to nondiabetic control subjects, and to investigate the relationship between the dispersion of the p wave and cardiac autonomic dysfunction in diabetic children. Methods: We enrolled 49 patients with type I diabetes, and 32 age- and sex-matched healthy subjects, measuring the Valsalva ratio, resting heart rate, and orthostatic hypotension in all. The duration of the p wave was measured manually on a high-resolution computer screen. Dispersion, defined as the difference between maximum and minimum durations of the p waves, was also measured in the 12-lead electrocardiogram before and after the Valsalva maneuver. Results: The mean age of the patients and their controls were 14.2 +/- 4.8 years, and 12.7 +/- 4.5 years, respectively. The mean duration of diabetes had been 6.2 +/- 4.6 years. Maximal and minimal values for the duration of the p wave were significantly decreased in the diabetic children, with the dispersion itself significantly increased. Values for the dispersion in the diabetic subjects were similar before and after the Valsalva maneuver, whereas dispersion was found significantly increased after this maneuver in the controls. The differences in the Valsalva ratio, resting heart rate, and orthostatic hypotension between the groups, on the other hand, were not found to be statistically significant. Conclusion: The noted increase in the dispersion of the p wave in diabetic children reveals the onset of cardiac electrophysiological heterogeneity before it is possible to detect parasympathetic and sympathetic dysfunction with other tests.